In general, guided-wave polarizer technology converts a circularly-polarized wave into a linear-polarized wave while maintaining orthogonality of the two possible senses of each polarized wave. For example, a guided-wave polarizer may convert a left-hand, circularly-polarized (LHCP) wave into a horizontal (H) linearly-polarized wave; alternatively, such a polarizer may convert a right-hand, circularly-polarized (RHCP) wave into a vertical (V) linearly-polarized wave. As is known in the art, such polarization conversion is based on decomposing circularly polarized waves into a superposition of two orthogonal, linearly polarized waves, in phase quadrature. Whether the composite field is LHCP or RHCP depends on which of the two linear components lags behind the other. A guided-wave polarizer advances or delays one of the field components by 90 degrees of phase relative to the other, bringing the two linear components into phase with one another, resulting in a linearly polarized composite wave. A guided-wave polarizer may also convert a linearly polarized wave into a circularly polarized wave, by the reverse process. Tolerances and errors in the conversion process typically result in some ellipticity of the wave, regardless of the desired polarization.
Many different structures have been developed to modify the polarization of a wave. One simple structure for converting from linear polarization to circular polarization is a hollow rectangular waveguide with a width that is slightly different from its height. A linearly polarized wave is introduced at a 45-degree angle relative to the waveguide; the wave is decomposed into two superimposed orthogonal linear TE10 modes (dominant modes) within the waveguide. As the two modes propagate through the waveguide, they will experience different cut-off frequencies and phase velocities as a result of the different width and height. The length of the waveguide is chosen such that one of the modes accumulates a 90-degree phase delay relative to the other mode across the length of the waveguide. The sense of the resulting circular polarization depends on the relative orientation of the linear polarization used to excite the two orthogonal modes, and the waveguide. Although this technique is relatively simple, only waves having a wavelength matched to the length of the particular waveguide will accumulate the 90-degree phase delay, resulting in a useful bandwidth of less than 1%.
Alternatively, as illustrated in FIG. 1A, another common approach is the use of a dielectric slab polarizer 100, such as described in U.S. Pat. No. 2,607,849 to Purcell et al. Polarizer 100 includes a hollow cylindrical waveguide body 110, formed of a conductive material, having a slab 120 of dielectric material disposed therein. In this case it is useful to consider the incident linear TE01 mode as the superposition of two orthogonal TE01 modes 151, 152, each at half the power of the composite mode. This places one of the component modes 151 parallel to slab 120 and the other component mode 152 perpendicular to slab 120. The parallel mode 151 strongly couples to slab 120, in which it experiences a reduced phase velocity that is inversely proportional to the square root of the dielectric constant of slab 120. The dielectric constant, thickness, and length of slab 120 are selected such that parallel mode 151 accumulates a total phase delay of 90 degrees with respect to the perpendicular mode 152 as the two modes propagate through polarizer 100.
One drawback of polarizer 100 is that differential phase shift induced by slab 120 monotonically increases with frequency. For example, FIG. 1B is a plot of the calculated differential transmission phase of the dielectric slab polarizer of FIG. 1A as a function of normalized frequency. To perform the calculation, the polarizer was modeled and its performance simulated using a High Frequency Structure Simulator (HFSS), commercially available from Ansoft (Pittsburgh, Pa.). Based on the calculation, it can be seen that the differential transmission phase increases monotonically with frequency. The bandwidth of polarizer 100 may be defined as the frequency range over which the differential transmission phase is within 90 degrees plus or minus some tolerance value, for example, plus or minus five degrees, divided by the center frequency of that frequency range. Using such a definition, the marginal performance that can be achieved with the dielectric slab polarizer of FIG. 1A is limited to a bandwidth of less than about 4-5%.
Another drawback of polarizer 100 is that parallel mode 151 must propagate within slab 120. As such, the dissipative loss of the parallel mode 151 will be greater than the loss of the perpendicular mode 152, because dielectric materials produce more Ohmic loss than conductive materials. Dielectric slab 120 is also susceptible to outgassing and to damage, requiring the power of the incoming wave to be maintained below the damage threshold of the dielectric material. Additionally, polarizer 100 may only meet performance requirements within a relatively narrow temperature range of operation, because (a) the dielectric constant of slab 120, and thus the accumulated phase delay of mode 151, varies with temperature, and (b) the coefficient of thermal expansion of slab 120 may be substantially different than that of cylindrical waveguide body 110, potentially damaging polarizer 100 if exposed to temperatures outside of an acceptable range. Furthermore, repeatability of the dielectric material properties and dimensions may be poor, causing performance to vary from polarizer to polarizer.
FIG. 2A illustrates another prior art waveguide polarizer 200, such as described in U.S. Pat. No. 2,546,840 to Tyrell. Polarizer 200 includes a hollow cylindrical waveguide body 210, formed of a conductive material. Polarizer 200 also includes first and second stepped ridges 221, 222, which are arranged opposite one another inside of waveguide body 210. Stepped ridges 221, 222 reduces the cut-off frequency and phase velocity of a first mode polarized in the y-direction relative to a second mode polarized in the x-direction, inducing a phase shift between the two modes. Each of stepped ridges 221, 222 includes three steps of varying heights, 261, 262, 263, which provide impedance matching for the incoming and outgoing waves. Steps 261 and 263 may have lengths of about ¼ of the guide wavelength of the mode propagating in the plane parallel to ridges 221, 222 to improve impedance matching. As illustrated in FIG. 2B, the calculated differential transmission phase between modes 251 and 252 within polarizer 200 decreases monotonically with frequency, yielding a useful bandwidth of only about 5%, assuming a tolerance of plus or minus five degrees about a ninety degree phase delay.
FIG. 3 illustrates another prior art waveguide polarizer 300, such as described in “Ridge Waveguide Polarizer with Finite and Stepped-Thickness Septum” by Bornemann et al., IEEE Transactions on Microwave Theory and Techniques, Vol. 43, No. 8, 1782-1787 (August 1995). Polarizer 300 includes a hollow square waveguide body 310, formed of a conductive material, and stepped septum 320 that bisects waveguide body 310. Stepped septum 320 has steps of increasing size along the length of polarizer 300; as described in Bornemann et al., the steps may also have increasing thickness. As orthogonal modes 351, 352 propagate along polarizer 300, mode 351 accumulates a 90-degree phase change relative to mode 352. Bornemann et al. report performance characteristics corresponding to a bandwidth of 21% for a ±5.4 degree (0.8 dB) phase variation from 90 degrees.
Polarization conversion can alternatively take place on an unguided, free-space wave with the use of multi-layer grids of linear or meander-line gratings. These structures tend to be relatively large and costly from a material standpoint.
Thus, prior art polarizers suffer from a number of deficiencies, including low bandwidth, high loss, low power handling capability, and/or large size.